1. Technical Field
The present invention relates generally to test circuits for evaluating the performance of circuits incorporated on a semiconductor die, and more particularly, to a ring oscillator circuit for determining delays of digital circuits.
2. Description of the Related Art
Ring oscillators have been commonly used in evaluation of digital circuits delay times. Since the propagation delay time of digital circuits on a semiconductor die can be very short, especially at a single-gate level, direct measurement is difficult or impossible. The measuring equipment accuracy is generally limited by the same constraints as the circuit under test, and will typically have resolution and accuracy limitations that do not even approach the delay time of a single gate integrated on a semiconductor die. The use of a ring oscillator allows a simple frequency measurement to provide useful information about the delay time of a digital circuit. By connecting a number of circuits in series to form an inverting chain and providing feedback from the output of the circuit under test back to the input of the circuit under test, a measurement of the frequency of the ring oscillator formed by the above-described structure yields a measure of the delay times through the circuit under test.
Dynamic circuits, such as domino logic circuits, are often evaluated by a ring oscillator technique. For dynamic circuits, the pre-charge control (clock) is typically used as the feedback point and other logic inputs are set so as to provide an enabled inverting path through the logic. U.S. Pat. No. 6,538,522 describes such a test circuit.
However, the ring oscillator described in the above-referenced patent includes design constraints that limit the range over which the evaluation delay time of the dynamic circuit under test can vary, while still providing an accurate measurement. In particular, if the evaluation delay time is very short compared to the pre-charge delay time, accuracy and resolution of a measured evaluation time is reduced due to the dominance of the pre-charge time in the determination of the ring oscillator output frequency.
Further, in dynamic circuits it is desirable to measure changes in the evaluation state due to leakage and other phenomena that may occur long after evaluation commences. For example, a dynamic circuit may evaluate, but the time required for the output to change to another state due to leakage determines the evaluation “delay time” due to leakage, i.e. the length of time for which the dynamic gate output can be considered reliable.
The circuits described in the above-referenced patent, if designed to measure long evaluation periods, cannot then measure short evaluation periods, as the one-shot used to produce an evaluate pulse to the circuit under test must have a pulse width that is less than the evaluation time in order for the ring oscillator to oscillate, but not so much less that the pulse disappears and the circuit pre-charges again before the circuit has evaluated. Thus, the circuit described in the above-referenced patent requires control of two parameters: the maximum one-shot time and the minimum evaluate pulse width, making control of the circuit over wide operating ranges difficult or impossible.
Therefore, in order to measure widely varying evaluation and/or pre-charge times, multiple ring oscillator circuits according to the above-referenced patent would have to be implemented using differing one-shot times in order to provide a wide range of measurement.
It is therefore desirable to provide a test method and ring oscillator circuit system for measuring dynamic circuit characteristics that provides a wide measurement range of evaluation time and/or pre-charge time using a single test circuit. It is further desirable to provide such a circuit and method that do not require control of multiple circuit characteristics in order to perform the measurement. It is further desirable to provide such a circuit and method that can also measure evaluation leakage times.